Survival of the fittest females / Tiny animals evolve through the ages without males' help

Published 4:00 am, Monday, March 22, 2004

The typically elongated Rotaria bdelloid rotifer reacts to stimulation by shrinking down to a compressed shape. Photos courtesy of Ron Neumeyer

The typically elongated Rotaria bdelloid rotifer reacts to stimulation by shrinking down to a compressed shape. Photos courtesy of Ron Neumeyer

Photo: John Blanchard

Survival of the fittest females / Tiny animals evolve through the ages without males' help

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Seventy-five years ago, in the title of their classic send-up of how- to books, James Thurber and E.B. White famously asked, "Is Sex Necessary?"

At last, an answer is emerging: No, sex isn't necessary at all.

So suggests a team of biologists after studying a major class of unisex animals that has managed to evolve into hundreds of different species without ever a male-female liaison. Stripped of their reproductive role, the males ceased to exist millions of years ago.

The animals, known as bdelloid rotifers, are among the most common microscopic creatures in the world, but they confound one of the classic assumptions of biology: namely, that if animals do not reproduce sexually, they are doomed to rapid extinction because they lack the possibility of beneficial gene forms arising from the random combination of male and female chromosomes.

The bdelloids (pronounced "dell-oids") are far from extinct. Molecular biologists at the Marine Biological Laboratory in Woods Hole, Mass., and Harvard University are seeking fresh evidence from the animals' genes to reveal how the creatures manage to adapt and evolve.

The three scientists published accounts of their work earlier this year in the Proceedings of the National Academy of Sciences and also in the journal Science, and they continue to work on their evolutionary studies with the animals.

There are no males in some 370 known species of bdelloid rotifers, and although every one of the tiny animals is a female, they have managed to evolve from species to species with their own genes apparently doing the entire job, the researchers are discovering.

Unlike sexless amoebas or other single-celled organisms that simply multiply by dividing, the bdelloid rotifers are true animals. Their different species can be found in ponds, in swamps, in running creeks and in puddles of rainwater and melted ice in springtime -- even in the dry valleys of Antarctica.

Their colorful semi-transparent bodies have heads covered with microscopic hairlike cilia that weave and spin; they have feet with claws; their mouths have jaws that crunch bacteria and algae particles, and their powerful stomachs digest their prey efficiently. And they have true gonads that produce eggs which hatch into new offspring, Jessica Mark Welch says.

All that in animal bodies that are less than a millimeter long -- smaller than the thickness of a sewing needle.

The eggs of the female bdelloid never need to be fertilized by competing males, so there's no possibility that what Charles Darwin called "sexual selection" could be operating in their evolution, Welch said in an interview.

Darwin gave the process of sexual selection a major role in his "Origin of Species," where he wrote that it "depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor but few or no offspring."

Seeking an explanation for the success of the bdelloids, the three scientists have determined the complete sequences of four crucial genes in four separate species of the animals. Among animals, including humans, sexual reproduction gives offspring two copies of all its parents' genes -- one copy coming from each parent. But the female bdelloid endows her offspring with two copies of her own genes, the researchers have discovered.

So in the past, over thousands or perhaps millions of years, tiny harmless mutations in those genes have accumulated to create entirely new species of bdelloids adapted to entirely new environments, according to the Welches and Meselson.

From the bodies of ancient bdelloid species preserved in amber, and from "molecular clocks" estimating the mutation rates of the animals, the researchers concluded that the tiny animals have been around for at least 50 million years, and perhaps 100 million.

In animals that reproduce sexually, those that survive environmental challenges typically are aided by "good" gene mutations that are passed along to the next generation. Individuals with "bad" gene mutations may fail to adapt quickly enough -- or survive long enough to reproduce. So extinction is their end, while the lucky survivors go on, sometimes evolving into completely new species.

But how does that process operate in the genes of the bdelloids, who inherit their genes from only one parent, not two? How have they diverged into so many separate species, adapted to so many different environments?

That's the mystery the Welches and Meselson are trying to solve by determining the sequences of many more genes in the all-female bdelloids, and by understanding their function. Thereby, perhaps, they hope to determine just why sex -- not copulation, but sexual reproduction -- exists at all.

"Twenty years ago," said Meselson in a telephone interview, "I asked myself, why does sex exist? What role does it really play in evolution? What good is sex doing for virtually all living creatures on Earth? And what is it that goes wrong in most asexual creatures that drives them to extinction -- except for the bdelloids? And now, in the laboratory, we're still hard at it.

"John Maynard Smith, the British biologist, called the existence of the bdelloids 'an evolutionary scandal,' and we're trying to resolve the scandal. We're trying to solve a major question in evolutionary biology."

Meselson said his team has a "hunch" that aside from the functioning genes of a bdelloid, there may be stretches of what geneticists call "junk" DNA containing snippets of a sequence that generates a mutation. Somehow that sequence endows the animal with the genetic ability to adapt to changing environments and thereby to evolve into new species, Meselson suggests.

"So what we need to do now," he said, "is to sequence the entire genome of the animals and hunt there for answers to our questions about sex. With the gene sequencing power of machines we have now, that's hardly a two-week job. But we're eager to try."

Seventy-five years ago, Thurber and White wrote: "There is an erroneous impression current nowadays that sex is everything. Nothing could be more stupid." The rotifer researchers are proving them right.